Progressively changing displays can be used both to perform timing functions and to display the progress or results of the timing functions. Such displays that are arranged to mark the passage of time are particularly useful as attachments to products with a limited life, such as food, room deodorants, flea collars, roach traps, and other products whose usefulness or effectiveness decreases over time. The progressive changes in these displays can also be equated to the service lives of other products such as batteries or filters, where service life is determined more by usage.
Various mechanisms have been used to effect such progressive changes in displays including physical migration, chemical reactions, and electrochemical reactions. Among the latter are displays that include electrochromic materials and voltaic or electrolytic cells.
For example, U.S. Pat. No. 4,804,275 to Kang et al. discloses a self-powered electrochromic timing device in which a color change boundary in an electrochromic material is advanced by a gradual dissolution of an electrode. Kang et al.'s electrochromic reactions, however, require a strong acid and other materials that add cost and pose problems for both manufacture and use.
U.S. Pat. No. 5,339,024 to Kuo et al. discloses a self-powered charge indicator cell connected in parallel with a main cell. An anode layer carried on a conductive substrate of the indicator cell is gradually oxidized (i.e., eroded) to reveal a message written in ink on an underlying layer. The thickness of the anode layer is tapered or stepped to regulate its rate of disappearance. Another embodiment arranges the anode and cathode layers side-by-side and fashions the electrolyte layer as a porous film straddling both electrode layers. The anode layer erodes under the electrolyte film in a direction away from the cathode layer. Both of Kuo et al.'s embodiments are subject to "islanding", however, where portions of the anode layer become electronically isolated from the cathode layer and prevent the anode layer's more complete disappearance.
U.S. Pat. No. 5,418,086 to Bailey discloses an electrolytic type battery charge indicator powered by the monitored battery. One electrode layer is dissolved and redeposited on another electrode layer as an indication of battery usage. The rate of dissolution and redeposition is controlled by tapering or stepping electrolyte layer thickness between the electrode layers. Like in Kuo et al., the dissolving electrode layer is also subject to islanding, which limits further dissolution of the electrode layer. Another embodiment positions the two electrode layers side-by-side on a common substrate and fills a space between them with electrolyte. The exchanges between electrode layers are expected to grow increasingly irregular with variations in distance between them. Also, the electrolysis operations are at least partially reversible, which can be a problem if more permanent change is desired.